Je prends un billet aller-retour.

Because prends is the 1st person singular form of the verb prendre in the present tense.

Present tense of prendre:

• je prends = I take / I’m taking
• tu prends
• il/elle/on prend
• nous prenons
• vous prenez
• ils/elles prennent

So with je, you must say je prends.

Yes, prendre literally often means to take, but in French it is also commonly used in contexts like booking, ordering, or choosing something.

In this sentence, je prends un billet aller-retour means something like:

• I’ll take a return ticket
• I’m getting a round-trip ticket
• I’m booking a round-trip ticket

So it does not always mean physically taking something in your hand.

Because billet is a masculine noun in French.

So:

• un billet = a ticket
• le billet = the ticket

If the noun were feminine, you would use une instead.

Both can refer to a ticket, but they are not used in exactly the same way.

Very generally:

• billet is common for train tickets, plane tickets, concert tickets, theater tickets, etc.
• ticket is often used for transport tickets, receipts/slips, or in certain fixed expressions.

For travel like this, un billet aller-retour is very natural.

Aller-retour literally combines:

• aller = to go
• retour = return

Together, aller-retour means:

• round-trip
• return

So un billet aller-retour is a round-trip ticket or return ticket.

Because aller-retour is a fixed compound expression in French, and it is normally written with a hyphen.

Examples:

• un billet aller-retour
• faire l’aller-retour = to go there and come back

So the hyphen is part of the standard spelling.

Because aller-retour directly describes the kind of ticket. It works like a fixed label after billet.

So French says:

• un billet aller-retour

not usually:

• un billet de aller-retour

You can think of aller-retour here as a set expression meaning return/round-trip.

Yes, it is natural, especially in a practical context like speaking at a ticket counter.

However, depending on the situation, French speakers might also say:

• Je voudrais un billet aller-retour. = I would like a round-trip ticket.
• Je vais prendre un billet aller-retour. = I’m going to get a round-trip ticket.
• J’achète un billet aller-retour. = I’m buying a round-trip ticket.

Of these, Je voudrais... is often the most polite when making a request.

In French, the noun usually comes first, then the expression describing it.

So:

• billet = ticket
• billet aller-retour = round-trip ticket

This is normal French word order. You would not say:

• aller-retour billet

because that does not fit standard French structure.

You would say:

• Je prends des billets aller-retour. = I’m getting round-trip tickets.

Notice:

• billet becomes billets
• aller-retour usually stays the same in this expression

So:

• un billet aller-retour
• des billets aller-retour

A simple pronunciation guide is:

zhuh prahn uhn bee-yay ah-lay ruh-toor

A few helpful notes:

• je sounds like zhuh
• prends has a nasal vowel, roughly prahn
• billet sounds like bee-yay
• in aller, the r is the French r
• retour sounds roughly like ruh-toor

A more IPA-style version is: /ʒə pʁɑ̃ œ̃ bijɛ ale ʁətuʁ/

Sometimes you may hear ticket in some transport contexts, but billet aller-retour is the safer and more standard choice for a travel ticket.

So for a learner, it is best to remember:

• un billet aller-retour = the normal phrase

No. In normal French, you need an article here.

So you say:

• Je prends un billet aller-retour.

not:

• Je prends billet aller-retour.

French usually requires an article before a singular countable noun like billet.